Tuned vibration absorbers (TVA) are secondary mass-spring systems that can be tuned to vibrate at a select frequency to absorb vibration occurring at that frequency. TVAs are used, for example, in aircraft fuselages to absorb the vibration caused by propeller aerodynamic wash impinging on the fuselage.
Examples of prior art tuned vibration absorbers are illustrated in FIGS. 1a, 1b and 2. The absorber illustrated in FIGS. 1a and 1b includes an annular elastomeric spring 20 bonded on a base plate 22. A tuned mass includes a large body 24 and a plurality of washers 26 fastened to the large body to fine tune the total mass. A shaft 28 extends through the center of the spring 20 and through holes in the large body 24 and the washers 26 to fasten these components together. The base plate 22 is mounted to a target structure 30 by two nut and bolt combinations 32. As shown, the absorber is paired with an identical absorber mounted on an opposite side of the target structure, which is a typical arrangement for mounting the absorbers in an aircraft frame. A mass vibrating at a distance from a rib (as in the absorber of FIG. 1) will produce a bending moment on the rib. The absorbers of the prior art are paired so that bending moments imparted to the rib by each absorber are cancelled by the paired absorber.
A cup-shaped cover 34, best illustrated in FIG. 1b, is mounted over the spring 20 and masses 24, 26 to isolate the spring and masses from external interference. The cover has mounting flanges 36 providing two linearly aligned mounting holes 38. A plurality of air holes 40 is provided in the cover top to allow condensate to escape.
FIG. 2 illustrates another prior absorber having a base 22 and a three-sided cover 22'. A mass 24 is bonded between two elastomeric springs 20. The spring 20 and mass 24 assembly is contained between metal end plates 23. A bolt 27 extends through the assembly and is fastened on opposite walls of the cover 22'. The system is tuned to a vibration frequency by adjusting the tension in the bolt and thus the compression in the spring. Four mounting holes 25 are used with nut and bolt combinations to fasten the device to a structure.
The prior art devices have deficiencies in manufacturing and performance. For example, the device of FIG. 1a mounts to a target structure with a mounting plate having two, linearly aligned mounting holes. Two holes are conventionally provided because of space constraints in aircraft frame structure, that is, the difficulty in finding a flat surface to mount vibration absorbers. In an aircraft frame, the stiffening rings are often additionally supported by secondary stiffening rings that leave little room for mounting a vibration absorber. The device is typically mounted so that the spring vibrates in shear, that is, perpendicular to the shaft 28 shown in FIG. 1a.
The inventor of the present invention has discovered that the prior vibration absorber is subject to a rocking moment about an axis A--A formed by the mounting bolts when the absorber vibrates at the disturbance frequency. The rocking moment can interfere with the device vibrating at the tuned frequency, and over time, can cause the bolts to loosen.
In the device of FIG. 2, the bolt 27 supplying tension to the spring and mass system is subject to vibration itself if not tightened correctly.
The present invention provides a tuned vibration absorber that is simpler to mount to an aircraft frame.
The tuned vibration absorber is designed with a low profile that minimizes the bending moment imparted to an aircraft frame stiffening rib, and as a result, can be mounted as a single, unpaired unit. This reduces the number of vibration absorbers needed in an aircraft, which accordingly reduces the cost.
The tuned vibration absorber with a four hole mounting plate also advantageously eliminates the problem of a rocking moment created by the prior mounting structure.
According to the invention, a tuned vibration absorber includes a mounting base having at least two holes arranged off the center axis of the spring mass system. The mounting holes may be alternatively provided as four holes in rectangular spaced relationship. The four mounting holes advantageously eliminate the rocking moment problem of the prior art.
According to another aspect of the invention, an elastomer spring is bonded to the mounting base and supports a main mass, which forms a spring and mass vibrating system. The elastomeric material is selected to have a resiliency to provide a spring constant appropriate for the tuned vibration frequency. The mass is fine tuned by attaching fine tuning masses, having a washer-like shape to the main mass. The fine tuning masses are attached to the main mass directly by rivets. Rivet fasteners provide sufficient fastening tightness and strength so that the main mass and fine tuning masses act as a single mass in cooperation with the spring.
According to another aspect of the invention, a cover to isolate the spring and mass system from external interference is formed from a single sheet of stamped metal bent into a box shape. The cover is easily formed to have gaps between the adjacent bent walls to prevent edge contact of the walls which may cause noise. Further, bending easily provides flanges, on which mounting holes to mate with the mounting base are provided.
The main mass and elastomer spring are shaped with rectangular outlines, which provides a center of gravity distance from the mounting plate that is smaller for the same mass than the cylindrically shaped mass of the prior art. By positioning the center of gravity of the mass closer to the mounting plate, and thus, the stiffening rib, any bending moment produced by the mass is minimized.
For mounting the vibration absorber to a stiffening ring of an aircraft, which is an advantageous location, the invention provides spacing plates to accommodate the thickness of a secondary stiffening ring that overlaps a main stiffening ring. The spacing plate is positioned on a main stiffening ring adjacent to the overlapping secondary stiffening ring to provide a flat surface for mounting the vibration absorber.